Minimum libcurl version

[skidoo23]

MPD commit 4efd0a9 makes use of CURLINFO_CONTENT_LENGTH_DOWNLOAD_T.

According to MPD/src/lib/curl/meson.build project requires libcurl 7.33 as minimum which is about nine years old. But CURLINFO_CONTENT_LENGTH_DOWNLOAD_T was introduced with libcurl 7.55 about five years ago. Should MPD raise its minimum libcurl version to 7.55 or do some magic to maintain compatible to older versions?

[MaxKellermann]

Raise minimum.
I changed to the new option because the old one emitted a deprecation warning with new curl versions. But after so many years, it's reasonable to require a newer version.